Magnetic resonance (MR) image quality is dependent upon the uniformity of the B1 transmit field. Toward this end, the radio frequency coils used for magnetic field excitation are designed to generate a substantially uniform B1 transmit field over the field of view (FOV). This can be done by using a plurality or array of radio frequency transmit coils that are strategically placed respective to the FOV so as to collectively generate a substantially uniform B1 transmit field over the FOV. Such design is imperfect, however. Moreover, a B1 transmit field designed to be uniform in the unloaded state can be distorted by magnetic susceptibility of the subject. This is known as the subject loading effect, and this effect becomes pronounced at higher static B0 magnetic field such as at static magnetic field of about 3 Tesla or higher. Even at lower static magnetic field, the subject loading effect may be non-negligible.
B1 shimming can be used to enhance uniformity of the B1 transmit field in either the unloaded or loaded state. Additionally or alternatively, B1 transmit field inhomogeneity can be compensated mathematically during post-acquisition MR image reconstruction processing. A spatial map of the B1 transmit field is an input for both B1 shimming and post-acquisition mathematical compensation. In view of the subject loading effect, the B1 transmit field is preferably mapped with the specific subject undergoing imaging loaded in the MR scanner and in the imaging position. In the alternative, the B1 transmit field can be mapped with a phantom suitably similar to the subject loaded in the MR scanner.
In existing B1 transmit field mapping MR sequences, measurements are performed for each individual transmit channel or coil in two- or three-dimensions. For an N-coil array where N indicates the number of coils, N transmit field mapping sequences are therefore performed. Unfortunately, these B1 transmit field mapping MR sequences are relatively slow, and undesirably lengthen the imaging session time.